In 1995, about 82% of the wood pulp consumed at US paper and paperboard plants was produced using the Kraft process. Although the proportion of pulp from this source is likely to decline as new processes come on line, it is expected that well over 50% of wood pulp production will still be produced in 2020 using the Kraft process.
In the Kraft pulp production process, a fibrous material, most commonly wood chips, are broken down into pulp in a digester under pressure in a steam-heated aqueous solution of sodium hydroxide and sodium sulfide, called white liquor. After cooking in the digester, the pulp is separated from the residual liquid called black liquor. Black liquor is an aqueous solution containing wood lignins, organic material, and inorganic compounds oxidized in the digester during the cooking process. It is concentrated and then burned in a recovery boiler to generate steam, which is used in the pulp mill for pulp cooking and drying, and other energy requirements. The material remaining after combustion of the black liquor, called smelt, is collected in a molten bed at the bottom of the boiler and discharged to a dissolving tank to be recycled into new white liquor.
Kraft chemical and energy recovery boilers, in which the black liquor is burned, are large and expensive, with capacities installed in the last 30 years for pulp mills typically exceeding 1000 tons of pulp per day. It is difficult economically to add small incremental units of boiler capacity, so the capacity of the chemical recovery boiler is often the factor limiting the capacity of the entire pulp mill.
The effective burning capacity of recover boilers is frequently determined by the processes governing the deposition of fume, intermediate sized particles, and carryover of partially burnt liquor/smelt drops on heat transfer surfaces of the steam and water tubes in the boiler, and the attendant plugging of gas passages between and around those pendant steam and water tubes. Much effort has been made and continues to be made to improving the understanding of the mechanism of particulate and vapor deposition on the tubes. However, there are still no reliable on-line methods for systematically detecting the presence and build-up rates of these deposits.
Various efforts to control the rate and quantity of deposits on the pendant tubes in the boiler have been undertaken in the past. These include adjustments to conditions of combustion, such as the nozzles that spray the black liquor into the combustion chamber, and the way air is introduced into the combustion chamber. They also include systems, such as soot blowers, for removing deposits on the tubes before they seriously impact the operation of the boiler. These control efforts are most effective when they are immediately correlated to the results they produce, but heretofore there has been no reliable method of determining directly the amount of deposits on the pendant tubes. Such control efforts have therefore necessarily been based on indirect measurements and considerations, and have usually yielded unsatisfactory results.
The severe environment of boilers, namely the high temperature, turbulent gas flow, particle laden atmosphere, and intensity of radiation have made it difficult to develop a sensing system for detection and control of deposition on pendant tubes in Kraft recovery boilers that would be economically viable as a commercial product. Attempts to use near-IR cameras for direct monitoring of pendant tube deposits have failed to reliably produce good images over the span of large boilers, and devices operating at longer wavelengths have been impractical for boiler-side use because of prohibitive expense and the need for reliable cryogenic cooling.
U.S. Pat. No. 4,539,588 entitled “Imaging of Hot Infrared Emitting Surfaces Obscured by Particulate Fume and Hot Gasses” issued on Sep. 3, 1985 to Peter C. Ariessohn and R. K. James discloses an improvement in the technology of the time, but operated in a wavelength region of 1.5-1.8 micron, which has a relatively high susceptibility to light scattering by particles in the boiler gas stream.
Thus, there has long been a serious need for a deposition detection system for recovery boiler pendant tubes to solve the unfulfilled requirement to monitor the degree and distribution of fume, intermediate sized particles, and carryover particle depositions on recovery boiler tubes.